Megaco (defined identically in IETF RFC3015 and ITU-T H.248) is a device control protocol assigning data stream resources between decomposed user and control planes in telecommunications/data systems. In other words, the Megaco defines a general framework for a physically decomposed media gateway, where the intelligence of the gateway is in a master node called the media gateway controller (MGC) and the actual switching and media transfer are performed in one or more slave nodes called the media gateway(s) or multimedia gateway(s). Herein the term media gateway is used. Megaco is used between the media gateway and the MGC for resource reservations, connection settings, media transformation settings, signal/event indications and processing, quality of service (QoS) settings, sending statistics information and signaling control.
The Megaco can be used, for example, in situations where a professional mobile radio or a private mobile radio (PMR) system uses VoIP connections. Professional mobile radio or private mobile radio (PMR) systems are dedicated radio systems developed primarily for professional and governmental users, such as the police, military forces, oil plants, etc. PMR services have been offered via dedicated PMR networks built with dedicated PMR technologies. This market is divided between several technologies—analog, digital, conventional and trunked—none of which has a dominating role. TETRA (Terrestrial Trunked Radio) is a standard defined by ETSI (European Telecommunications Standards Institute) for digital PMR systems. U.S. Pat. No. 6,141,347 discloses a wireless communications system which uses multicast addressing and decentralized processing in group calls.
One special feature offered by the PMR systems is group communication. The term “group”, as used herein, refers to any logical group of three or more users intended to participate in the same group communication, e.g. call. The groups are created logically, i.e. special group communication information maintained on the network side associates specific user with a particular group communication group. This association can be readily created, modified or canceled. The same user may be a member in more than one group communication group. Typically, the members of the group communication group belong to the same organization, such as the police, the fire brigade, a private company, etc. Also, typically, the same organization has several separate groups, i.e. a set of groups.
Voice traffic in a group, as seen by the users, consists of speech items (i.e. talkspurts) of more or less continuous speech coming from a specific user to one or more recipients. In the following speech items are used to illustrate items of all possible media streams.
A group call typically has a long duration (up to days) during which communication takes place quite infrequently and each interaction is typically short. The total active traffic may be, for example, only 15 minutes during a call. Each talk burst or speech item has an average length of 7 seconds in the existing PMR systems. Therefore, the radio channels or other expensive system resources cannot be allocated all the time, because the service becomes much too expensive. Group communication with a push-to-talk feature is one of the essential features of any PMR network overcoming this problem. Generally, in group voice communication with a “push-to-talk, release-to-listen” feature, a group call is based on the use of a pressel (PTT, push-to-talk switch) in a telephone as a switch: by pressing a PTT the user indicates his desire to speak, and the user equipment sends a service request to the network. The network either rejects the request or allocates the requested resources on the basis of predetermined criteria, such as the availability of resources, priority of the requesting user, etc. At the same time, a connection is established also to all other active users in the specific subscriber group. After the voice connection has been established, the requesting user can talk and the other users listen on the channel. When the user releases the PTT, the user equipment signals a release message to the network, and the resources are released. Thus, the resources are reserved only for the actual speech transaction or speech item.
One of the requirements for group communications in communications systems is that group communication requires traffic discipline: one talks and the others listen. Therefore the radio interface is of a semi-duplex type. Only one direction is active at a time. The communications system must be able to control that only one member speaks at a time in a group.
A problem with the semi-duplex group VoIP service provided by the Megaco is that there is no mechanism to transfer some common intelligence of the group from the media gateway controller to the media gateway. This requires continuous message exchange between the MGC and the media gateway thus causing delays and network load. For example, in order to decide which one of the users is allowed to speak at a time, the media gateway must ask the media gateway controller which one of the users is the allowed one every time the speaking turn changes